The present invention is directed to small-scale Mach-Zehnder interferometer (xe2x80x9cMZIxe2x80x9d) devices and structures. The present invention is also directed to digital optical switches.
An optical network, in its simplest representation, consists of an optical source, a destination, and a matrix of devices (e.g., fiber-optical cables, waveguides, cross-connects, amplifiers, etc.) for causing an optical signal generated by the source to reach a desired destination. Physical and geographic boundaries present no impediment to telecommunication, data communication and computing, all of which may utilize all or part of an optical network. Consequently, the number or sources and destinations, and the combinations of sources and destinations and the communication paths therebetween, may be nearly infinite. Optical switches are used in the optical network for facilitating the routing of an optical signal to its desired destination.
By way of example, FIG. 1 depicts a block diagram of a part of an optical component 1 comprising a plurality of optically interconnected optical devices 3 (e.g., switches, filters, etc.), shown in FIG. 1 as switches. As used herein, the terms xe2x80x9coptical componentxe2x80x9d and xe2x80x9ccomponentxe2x80x9d refer to any and all of a plurality of interconnected devices which may operate using any combination of optical, opto-electrical, and/or electrical technologies and which may be constructed as an integrated circuit. Devices 3 can be optically interconnected by waveguides 5. Various other optical, opto-electrical, and/or electrical devices may also be included in the optical component, as a matter of design choice. As used herein, the terms xe2x80x9copticalxe2x80x9d, xe2x80x9copto-electricalxe2x80x9d, and xe2x80x9celectricalxe2x80x9d devices may include, by way of non-limiting example, lasers, waveguides, couplers, switches, filters, resonators, interferometers, amplifiers, modulators, multiplexers, cross-connects, routers, phase shifters, splitters, fiber-optic cables, and various other optical, opto-electrical, and electrical devices. The optical component 1 and devices 3 depicted in FIG. 1 are merely illustrative.
Although a single wavelength of light can be transmitted through the network, in order to increase the network""s data-carrying capacity it is preferable to transmit multiple wavelengths of light at the same time. This is currently accomplished using techniques known as wave-division-multiplexing (xe2x80x9cWDMxe2x80x9d), dense WDM (xe2x80x9cDWDMxe2x80x9d), and ultra-dense wave-division-multiplexing (xe2x80x9cUDWDMxe2x80x9d).
The ability to separate one optical signal from a plurality of optical signals (or one wavelength from a plurality of wavelengths in an optical signal) propagating within an optical network becomes more important as the number of signals transmitted through a single optical fiber (or waveguide) increases. As optical transmission evolves from WDM to DWDM to UDWDM, and beyond, more and more data contained in a multi-wavelength optical signal is transmitted over the optical network. Optical filters are one component that may be used to extract a desired signal (i.e., a desired wavelength) at a particular point or location in the network and route that signal to its desired destination, while also permitting other signals to continue along the network.
Optical networks transmit data as pulses of light through waveguides in a manner similar to electrical networks, which send pulses of electricity through wiring. Transmitting an optical signal between waveguides, which may occur in various devices employed in an optical network, may require the optical signal to leave one waveguide and propagate through one or more materials (mediums) before entering another waveguide. It is likely that at least one of the devices will have an index of refraction different than the index of refraction of the waveguides (which typically have approximately the same refractive index). It is known that the transmission characteristics of an optical signal may change if that signal passes through materials (mediums) having different indices of refraction. For example, a phase shift may be introduced into an optical signal passing from a material having a first index of refraction to a material having a second index of refraction due to the difference in velocity of the signal as it propagates through the respective materials and due, at least in part, to the materials"" respective refractive indices. As used herein, the term xe2x80x9cmediumxe2x80x9d is intended to be broadly construed and to include a vacuum.
If two materials (or mediums) have approximately the same index of refraction, there is no significant change in the transmission characteristics of an optical signal as it passes from one material to the other. Accordingly, one solution to the mismatch of refractive indices in an optical switch involves providing an index matching or collimation fluid to offset any difference in refractive indices. Consequently, the optical signal does not experience any significant change in the index of refraction as it passes from one waveguide to another.
An example of this approach may be found in international patent application number WO 00/25160. That application describes a switch that uses a collimation matching fluid in the chamber between the light paths (i.e., between waveguides) to maintain the switch""s optical performance. The use of an index matching fluid introduces a new set of design considerations, including the possibility of leakage and a possible decrease in switch response time due to the slower movement of the switching element in a fluid.
In addition, the optical signal will experience insertion loss as it passes between waveguides. A still further concern is optical return loss caused by the discontinuity at the waveguide input/output facets and the trench. In general, as an optical signal passes through the trench, propagating along a propagation direction, it will encounter an input facet of a waveguide which, due to physical characteristics of that facet (e.g., reflectivity, verticality, waveguide material, etc.) may cause a reflection of part (in terms of optical power) of the optical signal to be directed back across the trench (i.e., in a direction opposite of the propagation direction). This is clearly undesirable because the reflected signal will interfere with the optical signal propagating along the propagation direction.
Reflection of the optical signal back across the trench also can create problems if the facets not only are coaxial, but also are parallel to one another. That arrangement forms a Fabry-Perot resonator cavity, which, under the appropriate circumstances, allows for resonance of the reflected signal, in known fashion.
Size is also an ever-present concern in the design, fabrication, and construction of optical components (i.e., devices, circuits, and systems) for use in optical networks. It is strongly desirable to provide smaller optical components so that optical devices, circuits, and systems may be fabricated more densely, consume less power, and operate more efficiently.
Currently, optical switches can be constructed using a directional coupler or a Mach-Zehnder interferometer (xe2x80x9cMZIxe2x80x9d), as is generally known in the art. Mach-Zehnder interferometers are known devices which take an input optical signal, split the signal in half (generally, in terms of optical power), direct the split signals along different optical paths, apply a phase shift to one of those split signals, recombine the signals and then feed those combined signals as a single signal to an output. The amount by which the phase of one of the signals is changed will, in known fashion, affect the nature of the output signal.
Conventional Mach-Zehnder interferometers shift the phase of light traveling along one of the interferometers in one of several ways. If the electro-optic effect is used, one of the interferometer arms is made from a medium having an index of refraction which changes in the presence of an applied electrical field. Similarly, if the electro-thermal effect is used, the interferometer has an arm made from a medium having an index of refraction that changes as the temperature of the material changes. In each of these devices, changing the index of refraction of one of the interferometer arms is comparable to changing that arm""s optical length, and results in a relative phase shift between the two split signals. In another known type of MZI, one of the two interferometer arms is actually longer (and thus, optically longer) than the other, and this also results in a relative phase shift between signals propagating in each arm.
In the electro-optic and electro-thermal type devices, the conditions for effecting optical switching in a device using a MZI, which operates by introducing a phase shift of up to xcfx80 (i.e., 180xc2x0) into at least a part of the optical signal, are defined by the equation:                               Δ          ⁢                      xe2x80x83                    ⁢          φ                =                  π          =                                                    2                ⁢                                  xe2x80x83                                ⁢                π                            λ                        ⁢            Δ            ⁢                          xe2x80x83                        ⁢            nL                                              (        1        )            
where xcex94xcfx86 is the maximum possible phase shift of xcfx80, xcex is the wavelength of the optical signal propagating in the device, L is the actual length of the device, and xcex94n is the change in refractive index effected by the application of a carrier signal, electrical field, or change in temperature to the device. Since the change in refractive index typically achievable for current optical devices is on the order of approximately 10xe2x88x923, the actual length of the device needed to introduce a maximum phase shift of xcfx80 must be at least 1 mm, and preferably longer. However, to achieve large-scale density integration, the actual length L must be reduced without sacrificing the ability to effect a xcfx80 phase shift, or at least substantially xcfx80 in an optical signal. Those two requirements are mutually exclusive.
If the phase is to be applied using a MZI device having different length arms, the light traveling through the longer arm has its phase shifted relative to the light passing through the other arm. This technique cannot, however, be used to make compact optical switches.
There exists a genuine need in the art for compact optical switches that can effect a 0-xcfx80 phase shift and which overcome the above-described shortcomings of the prior art. Preferably, such switches would combine small size and high actuation speed with low power consumption.
The present invention is directed in part to a digital optical switch having a movable phase shift element in one interferometer arm suitable for use in an optical network. The present invention is also directed to a Mach-Zehnder interferometer device (xe2x80x9cMZIxe2x80x9d) that can selectively apply a desired phase shift, such as xcfx80 or substantially xcfx80, to an optical signal propagating through one of the MZI""s arms. A device according to this invention may be more compact and more rapidly actuated than comparable known devices.
A MZI in accordance with this invention can be constructed by providing in one of the MZI""s two arms a phase shifter which imposes a predetermined phase shift upon light passing through that arm. The present invention combines the strong photon confinement characteristics of waveguides and the functionality of a Mach-Zehnder interferometer to provide a compact optical device suitable for large-scale (i.e., dense) integration.
More particularly, this invention is directed to improved Mxc3x97M switches which employ Mach-Zehnder interferometers to control optical signals. In one embodiment the MZI is part of a 1xc3x971 switch, and in another embodiment the MZI is part of a 2xc3x972 switch. As already explained, MZI devices operate by dividing an input optical signal into two signals, applying a phase shift to just one of those signals, and then recombining the two signals. The output will depend upon the magnitude of the phase shift applied. As noted previously, known switches of this type are larger than desired because the MZI devices used therein operate using techniques which thwart miniaturization.
Switches according to the present invention differ from known optical switches because of the unique MZI provided in accordance with the present invention. A MZI constructed in accordance with embodiments of the present invention includes a phase shifter in one interferometer arm. The phase shift element is selectively moveable into and out of an optical path defined by and through the interferometer arm so as to introduce a predetermined phase shift into an optical signal propagating in and through that interferometer arm. This arrangement dramatically reduces the size of the MZI as compared with conventional optical switches, which may employ the electro-optic, electro-thermal or asymmetric arms to introduce a phase change in an optical signal. A MZI using a phase shifter in accordance with this invention is far more compact than a MZI which uses those known techniques.
The present invention is particularly applicable to optical switches that are formed on integral planar optical substrates. Generally speaking, an integrated planar optical substrate refers to a relatively flat member having a supporting substrate and a number of layers of different materials formed thereon. The substrate and the different materials have particular optical qualities so that optically useful structures such as waveguides can be formed on the supporting substrate by suitable shaping or other processing.
As explained in greater detail below, this invention involves phase shifters constructed using small-scale fabrication techniques. This invention also encompasses phase shifters made using other fabrications techniques which result in comparable devices.
The present invention takes advantage of the extremely small mechanical actuators which can be assembled using small-scale fabrication techniques, and so significantly reduces the room needed on a chip for optical switches. These more compact switches require less chip space and so provide for denser integration of a plurality of optical devices in an optical component. This invention also takes advantage of the strong photon confinement properties of small-scale waveguides, such as are disclosed in U.S. Pat. No. 5,878,070 and 5,790,583. Together these developments facilitate construction of optical devices that provide the benefits and advantages of the present invention.
One embodiment of the present invention involves a Mach-Zehnder interferometer having a single input, a single output, and first and second arms extending along an optical path direction of the interferometer. One arm has a phase shifter disposed therein. When the phase shifter is actuated an optical signal propagating through the arm having the phase shifter will experience a phase shift relative to an optical signal propagating through the other arm.
In accordance with the present invention, a MZI may be constructed with a selectively moveable phase shift element in one interferometer arm. That phase shift element may be moved into and out of an optical path defined by and through that interferometer arm so as to introduce a phase shift into an optical signal propagating in and through that arm. In so doing, the phase shift element changes the optical length of that arm, when compared with the optical length of the other interferometer arm. By way of non-limiting example, the phase shift element may be generally rectangular or square.
While it is generally known to provide a MZI as an element of an optical switch, a MZI constructed in accordance with the embodiments of the present invention provides significant advantages over prior art MZI devices and optical switches. For example, the micron-scale of the MZI enables construction of smaller optical switches that consume less on-chip real estate. The power requirements of the MZI to effect a desired phase change in an optical signal are also significantly reduced when compared with prior art MZI devices.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the disclosure herein, and the scope of the invention will be indicated in the claims.